For the manufacturers of suitcases, luggage and cases, casing materials with lightweight, strong structure and low cost are in substantial demand. Generally speaking, one conventional casing material for suitcases or luggage is a composite metal, which is strong in structure but heavy in weight, also expensive in cost. Another conventional casing material is a composite plastic material, which is lighter in weight and cheaper in cost but poorer in structure compared with the composite metal. For example, an acrylonitrile butadiene styrene (ABS) material is a popular material for manufacturing suitcases, luggage and so on. The ABS material has a nature of wear-resistance but limited. In addition, the suitcases made of the ABS material are easily deformed and the edges of the suitcases are also easily broken, after the impact between the suitcases during transportation.